We are doing pilot studies of the anatomical, ultrasonagraphic, and biochemical responses of newborns to estrogen exposure, in order to plan a clinical study comparing the estrogenic effects of soy formula, which is high in phytoestrogens, with cow milk based formula and human milk. Plausible effects include 1) anatomic expression of estrogen activity: breast buds, labial size, vulval anatomy, scrotal rugae, and, by ultrasound, small ovarian cysts and possibly prostate volume and 2) biochemical evidence of activity: decrease in gonadotropins, decrease in estrone/estradiol synthesis. These studies began in Summer, 2003. Genistein and daidzein are estrogenic isoflavones found in soybeans. Although they are about 1/10,000 as potent as estradiol, serum concentrations are up to 4 orders of magnitude higher than estradiol in infants fed soy formula. Clinical trial data show increases in cholesterol synthesis consistent with an estrogen effect in infants fed soy formula vs. human milk or cow milk formula, and lowered response to polio vaccination, which might or might not be an estrogen effect. Anatomic, sonographic, and other biochemical evidence of estrogen exposure has not been looked for in infants fed soy. Whether such widely used products do or do not have clinically detectable estrogenic activity in infants, the group with the highest exposure per kilo, is important clinically; in addition, it may be a crucial test of the ability of laboratory measures of estrogenicity to predict "endocrine disruption" in humans. We have begun pilot studies to examine the natural history of estrogen responsive physiology in the newborn, and will use the results to develop a longitudinal study of the possible estrogenic effects of soy formula on infants. The two clinical sites began enrolling children in summer, 2003, and will enroll for 6 months. Phytoestrogen analyses will be done by FDA and CDC. Hormone analysis will be done by NICHD, and data handling and analysis will be directed at NIEHS.